(1) Field of the Invention
The present invention relates to a polymerizable vinyl compound having a polythioether skeleton, which has an excellent storage stability, more particularly, which provides a homogeneous polymer having a good stability against oxidation, and to a process for the preparation thereof.
The present invention also relates to an optical element having an excellent light resistance, which is made from the above-mentioned polymerizable vinyl compound having a polythioether skeleton.
(2) Description of the Related Art
Radical reaction between an olefin compound and a thiol compound is known as the reaction for the synthesis of a thioether, and polymerization of both components is also known wherein a polyfunctional olefin compound and a polyfunctional thiol are polymerized. For example, Otsu et al tried to react bismethacrylamide with dithiol and they found that polymerization by normal addition is advanced in the presence of a radical initiator or basic catalyst but polymerization by abnormal addition is advanced in the presence of an acidic catalyst Kogyo Kagaku Zasshi, 69, 2, 340 (1966) . Furthermore, Japanese Examined Patent Publication No. 53-28,959, Japanese Unexamined Patent Publication No. 57-125,025, Japanese Unexamined Patent Publication No. 57-130,572 and Japanese Unexamined Patent Publication No. 53-134,096 disclose polymerization reaction of a polyene compound and a polythiol compound in the presence of a radical initiator or under irradiation with active energy rays such as ultraviolet rays.
However, these conventional techniques involve the following problem. Namely, by the action of the thiol group as a chain transfer agent, polymerization is advanced even during storage in the dark place. Especially in the case where the polyene compound is a derivative of acrylic acid or methacrylic acid, vinyl polymerization of the polyene compound is liable to occur in addition to the radical addition reaction between the polyene compound and polythiol compound. Accordingly, the thiol group is often left in the product.
In the case where it is intended to finally obtain a polymerized cured product, in the abovementioned process, the polymerization conversion is reduced, and because of insufficient curing of the polymerized cured product, new problems of reduction of the hardness, reduction of the solvent resistance and brittleness arise. Moreover, this process has a problem in that because of the difference of the copolymerizability between the polyene compound and polythiol compound, a heterogeneous cured product is readily obtained when a molded article having a large thickness is prepared. This heterogeneity leads to formation of streaks which shape differs according to the conditions. Especially when the cured product is used as a homogeneous transparent resin, the appearance characteristics are drastically degraded. For example, when the cured product is used for an optical disc or the like, an erroneous operation is caused at the recording or reading step. Moreover, in the case of a spectacle lens or the like, since the heterogeneity is observed by the naked eye, there arise problems of reduction of the visual power and reduction of the commercial value.
In the case where a composition comprising a polyene compound and a polythiol compound is used as the polymerizable monomer composition, as pointed out hereinbefore, if the thiol group is left, the storage stability is degraded and undesirable phenomena occur due to the heterogeneity at the polymerization step and the problem of an unstability of the cured product against oxidation arises. A coating treatment with a reflection-preventing film is sometimes carried out for reducing the light reflection of the cured product. This coating treatment is ordinarily performed by vacuum deposition of SiO.sub.2 or Al.sub.2 O.sub.3 at a high temperature under vacuum. At this coating treatment, if the thiol group is left on the surface of the cured product, the surface undergoes oxidation, and coloration or surface roughening occurs. In some applications, the cured product is exposed outdoors. Also in this case, the remaining thiol group is oxidized, and coloration or surface roughening occurs.
In order to provide organic glasses, use of resins having a high refractive index has been tried. However, resins having a high refractive index, for example, polystyrene (refractive index =1.59) and polycarbonate (refractive index =1.59), are thermoplastic resins, and since they are molded usually by injection molding, the strain is often left after molding. Moreover, these resins have a poor solvent resistance.
As the means for overcoming these defects of the molding material and molding method, there has been proposed a process in which a resin having a high refractive index is prepared by cast-polymerizing a composition comprising a monomer having an aromatic ring and/or a halogen atom as the molecule-constituting unit. This process is disclosed in, for example, Japanese Unexamined Patent Publications No. 57-28,115, No. 57-28,116, No. 59-184,210, No. 60-7,314, No. 60-179,406, No. 60-217,301, No. 60-186,514, No. 60-166,307 and No. 60-103,301.
However, a resin having a high refractive index, prepared according to this process, is poor in the light resistance, and the resin has a problem in that if the resin is exposed outdoors for a long time, the resin is colored into yellow to brown by sunlight. In the field where prevention of coloration is required, for example, when the resin is used for a spectacle lens or the like, this problem reduces the commercial value. Moreover, when the deterioration is extreme, cracks are formed on the surface of the resin and fogging or whitening is caused to occur.
As another means for increasing the refractive index, introduction of a sulfur compound as the molecule-constituting unit has been tried. For example, Japanese Unexamined Patent Publication No. 57-158,213 discloses a polymer having a high refractive index, which comprises a thioester of acrylic acid or methacrylic acid as a comonomer. Although the polymer obtained according to this process has a high refractive index, the polymer is thermoplastic and is not satisfactory in heat resistance and chemical resistance. Moreover, a polymer having a diphenylsulfone structure (see, for example, Japanese Unexamined Patent Publications No. 57-28,118, No. 57-147,505 and No. 61-108,616), a polymer having a diphenyl sulfide structure (see, for example, Japanese Unexamined Patent Publication No. 60-26,010) and a polymer having a dibenzyl sulfide structure (see, for example, Japanese Unexamined Patent Publication No. 59-164,501) have been proposed. However, monomers used in these proposals have generally poor compatibility with other liquid monomers and their molding is difficult. Moreover, the obtained resins having a high refractive index possess poor light resistance and coloration is easily caused.
Since an aliphatic sulfide compound is excellent in the light resistance, for example, a sulfide compound having a low molecular weight is used as an antioxidant for a resin, and a sulfide compound having a high molecular weight is frequently used as a sealing material for windowpanes. Furthermore, it is reported that a high-molecular-weight sulfide compound obtained by radical-polymerizing a polyene compound/polythiol compound by a radical initiator or chemically active energy rays such as ultraviolet rays is used as a printing material, a paint, an adhesive, a molding material or the like (see Japanese Examined Patent Publication No. 53-28,959 and Japanese Unexamined Patent Publication No. 57-125,207).
However, if a polyene compound composed of an acrylic or methacrylic acid derivative is used in this process, since the reactivity of a composition comprising the polyene compound/polythiol compound is high, the storage stability is very low. Moreover, since homopolymerization of the vinyl group of the polyene compound and radical addition reaction of the polyene compound/polythiol compound are simultaneously advanced, the mercapto group of the polythiol compound is liable to remain. Because of this undesirable reaction, the crosslinking density is reduced at polymerization and curing of the polymer is insufficient. Therefore, new problems of reduction in hardness, solvent resistance and strength arise. Furthermore, in the field of a spectacle lens or the like where homogeneity and transparency are required, because of the difference of the reactivity between the polyene compound and polythiol compound, abrupt change of the refractive index in a narrow region, so-called "polymerization unevenness", often occurs, and the optical characteristic or appearance is drastically degraded. If the polymer is used as an optical disc material, an erroneous operation is caused at the recording or reading step because of these problems.
Thus, when a conventional composition comprising a polyene compound and a polythiol compound is used as the polymerizable monomer composition for an optical material, the storage stability is poor and the problem of "polymerization unevenness" cannot be solved.